C中malloc引起的内存泄漏

Question

我很难理解我的代码中出了什么问题。我的程序有一个 函数，它一次从文件中读取 8 个字节的数据。

char *read_64_bit_data_from_file(FILE *file, size_t *number_of_chars_read ){
    char *buffer = malloc(sizeof(char) *  8);
    *number_of_chars_read  = fread(buffer, 1, 8, file);
    printf("%s\n", buffer);
    printf("Length of buffer: %ld\n", strlen(buffer));
    printf("Number of chars read: %ld\n", *number_of_chars_read);
    return buffer;
}

函数调用如下：

...
size_t num = 0;
size_t *number_of_chars_read = &num;
char *message = calloc(sizeof(char), 8);
do{
    message = read_64_bit_data_from_file(file, number_of_chars_read);
}while(*number_of_chars_read==8);
...

现在，当我编译运行程序时，我得到以下输出：

Two road�B�z
Length of buffer: 14
Number of chars read: 8
s divergx<�z
Length of buffer: 14
Number of chars read: 8
ed in a x<�z
Length of buffer: 14
Number of chars read: 8
yellow wx<�z
Length of buffer: 14
Number of chars read: 8
ood, Andx<�z
Length of buffer: 14
Number of chars read: 8
 sorry I
Length of buffer: 8
Number of chars read: 8
 could n
Length of buffer: 8
Number of chars read: 8`

问题是写入缓冲区的字符有时会超过8个，虽然fread returns值8 每次。

现在，如果我使用 calloc 而不是 malloc 来分配 buffer，我将得到所需的输出 。以下是这种情况下的输出：

Two road
Length of buffer: 8
Number of chars read: 8
s diverg
Length of buffer: 8
Number of chars read: 8
ed in a 
Length of buffer: 8
Number of chars read: 8
yellow w
Length of buffer: 8
Number of chars read: 8
ood, And
Length of buffer: 8
Number of chars read: 8
 sorry I
Length of buffer: 8
Number of chars read: 8
 could n
Length of buffer: 8
Number of chars read: 8`

我使用带有 -Wall 和 -Wextra 标志的 GCC 编译器编译我的程序。编译器没有给出警告。

我运行程序通过valgrind。以下是输出：

==6051== Invalid write of size 1
==6051==    at 0x10917B: char_as_binary (in 
/home/saksham/Documents/DES/DES)
==6051==    by 0x1091FA: string_to_binary (in 
/home/saksham/Documents/DES/DES)
==6051==    by 0x108EE6: main (in /home/saksham/Documents/DES/DES)
==6051==  Address 0x521ecf7 is 5 bytes after a block of size 2 alloc'd
==6051==    at 0x4C31B25: calloc (in 
/usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==6051==    by 0x1090EE: char_as_binary (in 
/home/saksham/Documents/DES/DES)
==6051==    by 0x1091FA: string_to_binary (in 
/home/saksham/Documents/DES/DES)
==6051==    by 0x108EE6: main (in /home/saksham/Documents/DES/DES)
==6051== 
==6051== Invalid read of size 1
==6051==    at 0x10900A: initial_permutation (in 
/home/saksham/Documents/DES/DES)
==6051==    by 0x108FA9: encrypt (in /home/saksham/Documents/DES/DES)
==6051==    by 0x108EFD: main (in /home/saksham/Documents/DES/DES)
==6051==  Address 0x521ec60 is 0 bytes after a block of size 64 
alloc'd
==6051==    at 0x4C31B25: calloc (in 
/usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
==6051==    by 0x1091D3: string_to_binary (in 
/home/saksham/Documents/DES/DES)
==6051==    by 0x108EE6: main (in /home/saksham/Documents/DES/DES)
==6051== 
==6051== 
==6051== HEAP SUMMARY:
==6051==     in use at exit: 2,076 bytes in 172 blocks
==6051==   total heap usage: 186 allocs, 14 frees, 12,972 bytes 
allocated
==6051== 
==6051== LEAK SUMMARY:
==6051==    definitely lost: 2,076 bytes in 172 blocks
==6051==    indirectly lost: 0 bytes in 0 blocks
==6051==      possibly lost: 0 bytes in 0 blocks
==6051==    still reachable: 0 bytes in 0 blocks
==6051==         suppressed: 0 bytes in 0 blocks
==6051== Rerun with --leak-check=full to see details of leaked memory
==6051== 
==6051== For counts of detected and suppressed errors, rerun with: -v
==6051== ERROR SUMMARY: 93 errors from 2 contexts (suppressed: 0 from 
0)

我真的不明白为什么 malloc 会这样，以及如何使用 calloc 修复它。我相信我的程序有 UB（未定义行为）。

请帮助我了解我正在监督的内容以及我哪里出错了。

Answer 1

为了让C-string风格的函数起作用（比如strlen），你需要在buffer中预留一个字节给NUL-terminator。

你没有这样做，所以你的程序的行为正式是未定义。您的编译器没有接受这一点，但 Valgrind 是。它还会警告您缺少 free。考虑分配 9 个字节，将 buffer[8] 显式设置为 0，并确保您不尝试将超过 8 个字节读入 buffer.

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